Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokeshire, Wales

Thomas, Richard; Williams, Brian P.; Morrissey, Lance B.; Barclay, William; Allen, K. C.

doi:10.1002/gj.1053

Cited by 10 publications

(13 citation statements)

References 151 publications

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“…Further evidence linking the RCF to the FWF is given by the local abundance of Beaconites barretti burrows within both formations (this study; Morrissey and Braddy 2004). By comparison, only one example is seen in the Cosheston Group (Thomas et al 2006), and their occurrence is very rare within the correlative Senni and Brownstones formations elsewhere within the Anglo-Welsh Basin (Morrissey 2006). No mass-flow deposits however have been reported in the Gelliswick Bay Formation.…”

Section: Provenance and Significance Of The Ridgeway Conglomerate Formentioning

confidence: 79%

The Ridgeway Conglomerate Formation of SW Wales, and its implications. The end of the Lower Old Red Sandstone?

Hillier

Williams

2007

Geological Journal

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The Devonian Old Red Sandstone Ridgeway Conglomerate Formation crops out in Pembrokeshire, SW Wales. It was deposited as part of a dryland alluvial fan, axial fluvial valley deposystem. It conformably overlies the mid Lochkovian Freshwater West Formation and probably predates deposition of the Lower Cosheston Group Mill Bay Formation indicating an Early Devonian (latest Lochkovian to earliest Pragian) age, rather than a Middle Devonian age as suggested by previous workers. It therefore represents the youngest preserved formation of the Milford Haven Group south of the Ritec Fault. The Formation thickens drastically into the Ritec Fault, indicating its control on sedimentation. The half-graben topography initiated deposition of a hangingwall alluvial fan that was sourced from a southerly Lower Palaeozoic/Precambrian provenance within the present-day Bristol Channel. The Formation is heterolithic in nature, with deposits on the fan reflecting a mixture of processes. Conglomerates were deposited primarily by laterally extensive sheetfloods, and as bars in low-relief, laterally accreted channels. Sandstones were also predominantly deposited by sheetfloods. Gritty mudrocks in comparison demonstrate deposition by cohesive debris flows.The fan prograded northward and interfingered with a low-gradient, high-sinuosity fluvial channel system dominated by inclined and non-inclined heterolithic stratification. Thinly laminated mudstone and sandstone interbeds were deposited in ephemeral fan-toe and axial valley lakes that may have developed during sub-humid climatic episodes. The lacustrine heterolithic association has associated matgrounds and possible 'algal roll-up' structures. Calcretized peetee structures and root traces comprise a lake margin calcrete association.Fan gravels prograded into the axial fluvial valley during periods of increased sediment flux that may represent semi-arid conditions and/or episodes of tectonic activity. Calcretes of varying development were established in both the fan and axial valley zones. Calcretes with lower stages of development are more proximal to the Ritec Fault reflecting decreased soil residence times and high deposition rates within the axial valley. More strongly developed soil profiles on the fan may indicate sequence boundaries associated with low sediment flux, or increased soil residence time due to active fan-channel migration (the pedofacies concept). Groundwater calcretes have sharp-based and layer-bound calcrete profiles. Gully-bed cements are locally developed within the fan gravels.

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Section: Provenance and Significance Of The Ridgeway Conglomerate Formentioning

confidence: 79%

The Ridgeway Conglomerate Formation of SW Wales, and its implications. The end of the Lower Old Red Sandstone?

Hillier

Williams

2007

Geological Journal

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“…Rossetti and Góes, 2000;Moretti, 2000;Onasch and Kahle, 2002;Moretti et al, 2002;Paz and Rossetti, 2005;Rodríguez-López et al, 2007;Fortuin and Dabrio, 2008). The stratigraphic distribution of seismites in between undisturbed layers records the timing of seismic events of sufficient magnitude to cause deformation (Sims, 1973;ElIsa and Mustafa, 1986;Pratt, 1994Pratt, , 2001bRodríguez Pascua et al, 2003;Migowski et al, 2004;Weidlich and Bernecker, 2004;Marco and Agnon, 2005;Paz and Rossetti, 2005;Thomas et al, 2006). For the purposes of gauging their distribution in the Herald Formation, individual deformed intervals are taken here to record single earthquake events.…”

Section: Seismic Originmentioning

confidence: 98%

Syndepositional tectonic activity in an epicontinental basin revealed by deformation of subaqueous carbonate laminites and evaporites: Seismites in Red River strata (Upper Ordovician) of southern Saskatchewan, Canada

Taki

Pratt

2012

Bulletin of Canadian Petroleum Geology

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The Yeoman and Herald formations of the Late Ordovician Red River succession of southeastern Saskatchewan comprise two carbonate-evaporite cycles in the epicontinental Williston Basin. Although these units possess an overall 'layer-cake' aspect across a broad area, thickness variations in the Herald Formation show that accumulation was affected by syndepositional flexure, differential subsidence, and displacement of basement faults thought to have been oriented orthogonally NE−SW and NW−SE. The dolomudstones and anhydrites of the Lake Alma and Coronach members host a variety of synsedimentary deformation features -seismites-interpreted to have been induced by earthquakes generated by movements of these faults, and the low-energy, subaqueous environmental setting was conducive to their preservation.Individual deformed intervals are <0.2 m thick. Folds and small-scale pinchand-swell structures indicate plastic behaviour of both carbonate and evaporite sediment under compression and shear stress. Brittle failure of stiffer material from compression or extension is shown by microfaults, cracks, disrupted layers, pseudo-intraclasts, breccias and cataclastites. Dikelets in interlaminated dolomudstone and anhydrite record cyclically elevated pore pressure that caused dewatering and shrinkage with concomitant fluidization and injection of carbonate mud and primary granular gypsum. Enterolithic anhydrite exhibits compressed vertical dikes that likely represent injection of granular gypsum. Clay-filled shearvein arrays occur rarely in dolomudstone. Seismites are absent in bioturbated limestones except for one horizon of molar-tooth structure.The different kinds of seismites are related to the varying rheology caused by the primary nature of the carbonate and evaporite sediment plus modifications due to deformation-induced dewatering, early diagenesis, and confinement under shallow burial, as well as earthquake magnitude. A minimum shaking intensity of MM=VI was probably required, meaning that events of lesser magnitude were not recorded. Some seismite-bearing horizons can be correlated across the study region. Within the limitations of core control, their geographic distribution varies, probably in relation to the basement faults. Seismites thus serve as proxy signals of syndepositional tectonic activity but also point to components of the basement structural fabric that, if reactivated later, might induce fracture porosity or influence subsurface fluid flow. Syndepositional tectonism undoubtedly had a much more profound influence on many successions than is presently thought, and its effects are more widespread than currently appreciated. RésuméLes Formations de Yeoman et d'Herald de la succession de Red River de l'Ordovicien tardif du sud-est de la Saskatchewan comprennent deux cycles carbonatés et évaporitiques dans le bassin épicontinental de Williston. Bien que ces unités présentent l'aspect général d'un gâteau étagé sur un large secteur, l'épaisseur variable de la Formation d'Herald révèle que les couches ont été tou...

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“…The landscapes were characterized by complex hydrology that resulted in mosaics of microenvironments that were potential habitats for early land plants. In conjunction with other palaeogeomorphological models for formations across the Anglo-Welsh Basin (Love & Williams 2000;Owen & Hawley 2000;Marriott & Wright 2004;Williams & Hillier 2004;Thomas et al 2006;), a basin-wide transition in palaeohydrology and geomorphology has been recognized between the Přídolí and Lochkovian-Pragian.…”

Section: Dynamic Lower Old Red Sandstone Landscapesmentioning

confidence: 88%

“…Models for late Lochkovian to Pragian landscapes across the basin envisage muddy floodplains incised by sandy to gravelly, meandering to braided perennial channels during periods of high discharge (Owen & Hawley 2000;Thomas et al 2006). It can be inferred from plant preservation within reduced stacked channel complexes that the water table was elevated for prolonged periods.…”

Section: Dynamic Lower Old Red Sandstone Landscapesmentioning

confidence: 99%

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Siluro-Devonian landscapes of southern Britain: the stability and nature of early vascular plant habitats

Morris¹,

Wright

Edwards³

2012

JGS

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A prerequisite for plant taphonomy and palaeoecological analysis of early land plants is to understand the palaeogeomorphology of the landscapes that they inhabited. The Lower Old Red Sandstone of the Anglo-Welsh Basin was chosen to ascertain the nature of the landscapes and range and variability of potential plant habitats. Evidence is provided for dynamic, low-lying landscapes, with complex hydrology and mosaics of microenvironments. The Raglan Mudstone Formation (latest Přídolí–earliest Lochkovian) represents a mud-dominated, ephemeral dryland river system, active during short-lived high-discharge events. Plant habitats were restricted to areas with temporarily elevated water tables, suitable for plants with short life cycles (e.g. rhyniophytes). The St. Maughans Formation (early Lochkovian) represents a sand-dominated, perennial trunk channel river system, with an overall wetter, more stable landscape. Plant habitats extended into areas of permanently elevated water tables, where plants with a more extensive vegetative growth stage survived (e.g. zosterophylls). In association with evidence from the plant fossil record, this leads to the hypothesis that during the latest Silurian to earliest Devonian the landscapes across the southern margins of Laurussia were too hostile (overall moisture deficient and unstable) for plants of higher organization than rhyniophytes to establish, despite their radiation across palaeoequatorial latitudes much earlier.

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Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokeshire, Wales

Cited by 10 publications

References 151 publications

The Ridgeway Conglomerate Formation of SW Wales, and its implications. The end of the Lower Old Red Sandstone?

The Ridgeway Conglomerate Formation of SW Wales, and its implications. The end of the Lower Old Red Sandstone?

Syndepositional tectonic activity in an epicontinental basin revealed by deformation of subaqueous carbonate laminites and evaporites: Seismites in Red River strata (Upper Ordovician) of southern Saskatchewan, Canada

Siluro-Devonian landscapes of southern Britain: the stability and nature of early vascular plant habitats

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